1. Field of the Disclosure
Embodiments disclosed herein relate generally to two or three component compatible polyolefin compounds having sufficient adhesion required for various applications. In another aspect, embodiments disclosed herein relate to two or three component compatible polyolefin compounds having sufficient adhesion and shock absorption useful in footwear applications. In yet another aspect, embodiments disclosed herein relate to a method of producing two or three component compatible polyolefin compounds and various composites using those compounds, having sufficient adhesion and shock absorption.
2. Background
Athletic shoes can produce a substantial impact stress on the foot when the foot strikes the ground. The stress is particularly intense in those sports that are practiced on artificial or other hard surfaces, such as track and field, basketball, volleyball, tennis, football, soccer, and numerous other sports. To cushion the impact stress on the foot, the sole of shoes designed for such activities includes one or more shock absorbing layers. The greatest need for shock absorption is in the zones of the foot where the stresses are greatest, which normally correspond to the bearing point of the heel. The primary objectives of a shock-absorbing sole include providing a better comfort and the least sensation of fatigue for such activities as walking, running, and jumping, while at the same time ensuring good shock absorption to protect the foot.
Most footwear articles include two primary elements, an upper and a sole structure. The upper provides a covering for the foot that securely receives and positions the foot with respect to the sole structure and is usually attached to one or more parts of the shoe sole.
The shoe sole structure typically incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole is a thin soft solid or foamed comfort-enhancing member located in the upper portion of the sole, adjacent to the plantar (lower) surface of the foot to enhance footwear comfort. The midsole forms the middle layer of the sole structure and serves a variety of purposes that include controlling foot motions and attenuating ground reaction forces via shock absorption. The outsole forms the ground-contacting element of the footwear and is usually fashioned from a durable, wear-resistant material that includes texturing to improve traction.
The primary element of a conventional midsole shock absorbing layer is a resilient polymer material, such as foamed polyurethane, foamed crosslinked EVA (ethylene vinyl acetate), or other foamed crosslinked polyolefin elastomer, which extends throughout the length of the footwear. The properties of the shock absorbing layer are primarily dependent upon factors that include dimensional configuration of the midsole and the specific characteristics of the polymer material. By varying these factors throughout the midsole, the relative stiffness, degree of ground reaction force attenuation, and energy absorption properties may be altered in order to meet the specific demands of the activity for which the footwear is intended to be used.
New casual and athletic shoes are using a different shock absorption design approach, eliminating traditional foamed midsoles and replacing them by structural injection molded arched parts that provide an appealing looks as well as most of the shock absorption requirements. In the absence of a traditional foamed midsole, these new designs usually require a thicker insole layer to maintain overall shoe comfort. A number of material properties must be considered in the design of thermoplastic compounds for thermoplastic injection molded shock absorbing systems, including, but not limited to: general mechanical strength, dynamic deformation, thermal resistance, adhesion, and optics. However, in many applications, a single polymer cannot provide optimal material properties in all the required performance areas.
To remedy the performance shortcomings of individual polymers and the resulting structural injection molded designs, specially engineered polymer compounds can be developed using various techniques, including copolymerization and polymer compounding. In formulating the polymer compounds and compounds, for example, for athletic shoe shock absorbers, various material selection trade-offs must be made that directly impact the overall shoe design features, including functionality, weight, and aesthetics. For example, an increased thickness of the midsole to increase the shock and energy absorption diminishes the shoe's lightweight properties, stability, and visual appeal.
In general, shock absorbers for athletic shoe soles are made from materials exhibiting good flexibility, resiliency and dynamic deformation. As previously discussed, these properties are required to provide foot protection from impact stresses and to elastically recover a portion of the energy as rebound force while walking. In addition, as the shock absorbing midsole layer or system is typically placed between other layers or surfaces, such as the outsole, the insole, or the shoe upper, the shock absorbing layer must have good adhesion to the other shoe layers. The polymer compound or blend must also have good thermal resistance to ensure that mechanical and dynamic properties are not sacrificed throughout a range of potential exposure temperatures and that the polymer does not degrade. Further, the polymer compound or blend may require adequate optical properties, such as transparency, to provide aesthetic appeal.
Polymer compounds from a group commonly referred to as thermoplastic elastomers (TPE) are frequently used in various shock absorbing applications, for example, in forming solid structural injection molded shock-absorbing shoe systems. TPE's may exhibit both plastic and elastic behavior, and thus may provide various advantages over other polymers and composites. Six general classes of TPE's include: polyolefin elastomers (POE), thermoplastic polyurethanes (TPU), styrenic block copolymers (SBC), elastomeric alloys, polyester elastomers (PEE), and thermoplastic polyamids.
TPU's are frequently used in shock absorbing applications, for example, in athletic shoes. In general, polyurethanes may be formed by reacting a diisocyanate, a monomer containing at least two isocyanate functional groups, with a polyol, a monomer containing at least two alcohol groups. TPU's typically possess high resiliency, and may be formulated to have a wide range of stiffness, hardness, and density.
PEE's are another type of elastomers widely used in shock absorbing structural applications. In general, polyesters contain the ester functional group, and may be formed via an esterification reaction.
High performance materials like TPU's and PEE's are frequently used in various footwear items. However, such high-performance elastomers are relatively expensive and are overdesigned for most applications that do not require top mechanical strength or dynamic deformation properties.
Thus, there is still a significant need for high-value polymer compounds that provide good shock absorbing performance in footwear applications at lower cost to compete with the traditional high-cost performance materials, such as TPU's and PEE's.